1. Field of Invention
The present invention relates to a wafer polishing apparatus, and more particularly, to an improvement for increasing uniformity of a polishing amount of a wafer surface.
2. Description of Related Art
As a wafer polishing apparatus of this type, an apparatus has been widely known which comprises a disc-like platen having a polishing pad affixed to the surface thereof; a plurality of wafer holding heads holding one surface of the water to be polished to abut the other surface of the water against the polishing pad; and a head driving mechanism for relatively rotating the wafer holding heads with respect to the platen, and which performs polishing by supplying a slurry containing polishing powders between the polishing pad and the wafer.
As an improvement to the apparatus of this type, U.S. Pat. No. 5,205,082 discloses a water holding head as shown in FIG. 6. The wafer holding head includes a hollow head body 1, a diaphragm 2 stretched in the horizontal position within the head body 1, and a carrier 4 fixed to the lower surface of the diaphragm 2. This water holding head is of a floating head construction in which the carrier 4 can be pressed downward by supplying pressurized air from a pressurized air source 10 to an air chamber 6 formed by the diaphragm 2. Such a floating head construction has an advantage in its capability to equalize an abutting pressure of the wafer against the polishing pad.
A retainer ring 12 is arranged concentrically on the outer periphery of the carrier 4. The retainer ring 12 is also fixed to the diaphragm 2. The lower end of the retainer ring 12 protrudes downward below carrier 4, thereby holding the outer periphery of the wafer adhered to the lower surface of the carrier 4. Thus, by holding the outer periphery of the wafer, the problem in which the wafer is removed from the carrier 4, can be prevented. In addition, it is believed that a phenomenon in which a polishing amount at the outer peripheral portion of the wafer becomes larger than that of at the center portion of the wafer can be prevented by polishing the wafer with the wafer surrounded by the retainer ring 12, and with the lower end of the retainer ring 12 allowed to be flush with the lower surface of the wafer.
Conventionally, it has been considered that an excessive polishing of the outer peripheral portion of the wafer can be prevented simply by arranging the lower end surface of the retainer ring 12 to be substantially flush with the surface of the water to be polished, as described above.
However, as a result of an extensive study made by the present inventors on the wafer polishing apparatus, a novel phenomenon is found in which, with some materials of the polishing pad, a polishing pad P locally swells along the inner peripheral edge of a portion thereof abutted against the retainer ring 12 (hereinafter, referred to as "waving deformation"), as shown in FIG. 7. Accordingly, an outer peripheral portion G of a wafer W is excessively polished by this swelling portions T, whereby the polishing uniformity of the wafer W is hindered. In addition, it is also found by the present inventors that when the phenomenon occurs, the above-described waving deformation can be prevented by setting an abutting force of the retainer ring 12 against the polishing pad P to a proper value which is smaller than conventially known, thereby substantially preventing the excessive polishing of the outer peripheral portion G of the wafer.